During the treatment of granular solids such as sulfidic zinc ore, iron ore, sponge iron or aluminum hydroxide in a fluidized-bed tank, the solids are fluidized by supplying a fluidizing gas and are treated, for instance, roasted, calcined, heated, reduced, burnt, gasified or cooled, corresponding to the desired application. In the case of the circulating fluidized bed, a recirculation cyclone regularly is provided downstream of the fluidized-bed reactor, from which the flue gases are conducted to the top, while the treated solids are recirculated downwards into the fluidized-bed tank. A partial stream of the solids is branched off as product or residue and possibly supplied to a desired further treatment stage.
DE 31 07 711 A1 describes a method for producing cement clinker, in which raw cement powder, which has been preheated in a suspension-type heat exchanger, is supplied to a circulation system for calcination. The circulation system comprises a fluidized-bed reactor, a recirculation cyclone, and a return conduit. In the fluidized-bed reactor, the cement raw meal is fluidized by means of fluidizing gas and calcined by combustion of a fuel introduced through a lance. Upon separation of the solids in the recirculation cyclone, a continuous stream of material is withdrawn via a aperture blocker-controlled withdrawal device and is supplied to a second circulation system, which in turn includes a fluidized-bed reactor, a recirculation cyclone and a return conduit. In the second circulation system, clinkering is effected by heating with a comparatively small amount of fuel.
The so-called aperture blocker used in the withdrawal device is a mechanical solids valve in the form of a lance with a conical tip which fits into a corresponding conically shaped opening of the tank wall. By withdrawing or inserting the lance into the opening, the cross-section is increased or reduced, so that the outflow can be controlled. However, the same pressure exists on both sides of the solids outlet, because the aperture blocker can effect a pressure seal only in the completely closed condition. In general, this will be the pressure of the fluidized bed at the level of the solids outlet. If a differential pressure over the solids outlet is obtained as a result of the switching of the process and/or the respective operating condition, a deterioration of the control quality must be expected.
EP 0 488 433 B1 describes a control aperture blocker for opening and closing a gas passage.
Such control aperture blockers are functioning in practice, but they have their weaknesses and disadvantages. On the one hand, the control aperture blocker has mechanically moving parts, which are in contact with hot solids. Therefore, it must be cooled by water cooling. Here, the flow rate of the cooling water and the temperature difference between forward flow and return flow must be monitored. Occasionally, a damage of the lance occurs. Then, water escapes from the lance and, in the worst case, flows into the tank located below the same, which has a refractory lining, whereby said refractory lining can be damaged. In addition, the lance must be moved laterally, with the drive on the outside having ambient pressure and the interior typically having excess pressure. For sealing purposes, a stuffing box is used. If the same becomes leaky, hot solids probably will be discharged, which represents a safety hazard, or ambient air will enter, which can disturb the process. To adjust the stream of solids discharged via the aperture blocker, an exact adjustment is required between the tip of the lance and the nozzle stone acting like a valve seat. It should be considered here that after extended operating periods the high temperatures can effect a displacement of the refractory lining, so that this exact adjustment can get lost. It can also occur that after an extended period with closed aperture blocker, the solids are defluidized before the tip of the aperture blocker and do not move upon opening the aperture blocker. In many cases, a manually handled air lance, which is moved through another stuffing box, can then be used for poking and at the same time fluidizing the solids. The success or failure of such poking typically can be observed through an inspection glass. When the solids are so hot that they are glowing, something can be seen. But if they are cold, nothing can be seen and one is working blind, so to speak. In the case of hot solids, however, the inspection glass withstanding the high temperatures is very expensive. Moreover, with a control aperture blocker a pressure seal cannot be realized by means of the control device. This can lead to gas/air flows through the nozzle stone, in the worst case also against the direction of the solids flow, whereby the solids flow can be delimited or even be inhibited completely.
Another disadvantage of such aperture blockers consists in that they only function in a downward direction, because gravity is required to move the solids horizontally through the opening of the nozzle stone.
U.S. Pat. No. 6,666,629 describes a method for conveying granular solids in which the solids are conveyed by means of a gaseous medium from a first zone with a pressure of 4 to 16 bar through a descending conduit and then via an ascending conduit to a second zone with a pressure lower than in the first zone by 3 to 15 bar. The inflow of the gaseous medium is effected through an upwardly directed nozzle at the point where the descending conduit opens into the ascending conduit.
WO 01/28900 A1 describes an apparatus in which solids are passed through a downer to an ascending conduit, through which they are conveyed by means of conveying gas and then are withdrawn at the bottom upon deflection. The solids are fluidized both in the descending conduit and in the riser along the entire length thereof and thereby are conveyed by gravity like a fluid in communicating tubes.
US 2005/0058516 A1 describes an apparatus for the transport of fine-grained solids with a controlled flow rate, wherein the solids initially flow downwards through a downer as a result of gravity and then are transported to a riser via an inclined transfer conduit by injecting a secondary gas, in which riser air is introduced from below, in order to convey the particles to the top. The downer and the riser accordingly are not directly connected with each other.
The last-mentioned methods and apparatuses have in common that the stream of solids is not divided.